PCCP
View Article Online
Open Access Article. Published on 20 April 2015. Downloaded on 06/02/2017 10:22:27.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
PAPER
Cite this: Phys. Chem. Chem. Phys.,
2015, 17, 13559
View Journal | View Issue
A density functional theory insight towards the
rational design of ionic liquids for SO2 capture†
Gregorio Garcı́a,a Mert Atilhanb and Santiago Aparicio*a
A systematic density functional theory (DFT) analysis has been carried out to obtain information at the
molecular level on the key parameters related to efficient SO2 capture by ionic liquids (ILs). A set of 55 ILs,
for which high gas solubility is expected, has been selected. SO2 solubility of ILs was firstly predicted
based on the COSMO-RS (Conductor-like Screening Model for Real Solvents) method, which provides a
good prediction of gas solubility data in ILs without prior experimental knowledge of the compounds’
Received 6th January 2015,
Accepted 16th April 2015
features. Then, interactions between SO2 and ILs were deeply analyzed through DFT simulations. This
DOI: 10.1039/c5cp00076a
solubility in ILs, which is crucial for further implementation of these materials in the future. In our opinion,
www.rsc.org/pccp
controlled at the molecular level, providing an approach for the rational design of task-specific ILs.
work provides valuable information about required factors at the molecular level to provide high SO2
systematic research on ILs for SO2 capture increases our knowledge about those factors which could be
1. Introduction
Air pollution is attracting increasing attention throughout the
world. Among the main air pollutants, sulfur dioxide (SO2), which
is mainly emitted through the combustion of fossil based fuels, is
causing serious harm to the environment and human health.1,2 At
the same time, SO2 is a useful source of many intermediates in
chemical synthesis.3 As a matter of fact, there is general interest in
the design and improvement of methods for SO2 capture. Although
several methods have been developed for this purpose, all of them
have several drawbacks. For instance, an effective method based on
flue gas desulfurization (FGD) needs a large amount of water and
subsequent treatment of the consequent waste, in order to prevent
excessive amounts of calcium sulphate that lead to secondary
pollution in the environment. Other methods, such as amine
scrubbing, are affected by solvent loss and degradation due to
the low volatility and stability of amine solutions.2,4,5
In recent years, ionic liquids (ILs) have demonstrated their
effectiveness for acid-gas removal from flue gas such as SO22,3,5–9
and CO2.8–14 In addition, ILs contain unique properties, including good thermal and chemical stability, non-flammability and
most distinctly they have almost null vapor pressure. All these
features have been proved to be useful in chemical processes to
replace volatile organic compounds. Nonetheless, the major
a
Department of Chemistry, University of Burgos, 09001 Burgos, Spain.
E-mail: [email protected]
b
Department of Chemical Engineering, Qatar University, P.O. Box 2713, Doha,
Qatar
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5cp00076a
This journal is © the Owner Societies 2015
advantage of ILs is the possibility to design task-specific solvents
through the adequate cation–anion combinations, which requires
a deep understanding of the structure–property relationship.9,15
There is a large collection of compounds (approximately about
B106 when considering only ‘‘pure’’ ILs), and thus, system
approaches on the ability of ILs for acid gas capture are useful
in the selection of ILs for SO2 storage. Unfortunately, the larger
number of ILs hinders systematic experimental studies on a huge
number of ILs, due to the economical and temporal cost as well as
limited experimental resources. Having mentioned the cost of
experimental difficulties and cost hurdles associated with broad
screening of ILs for acid-gas removal, density functional theory
(DFT) simulations have proven their ability to provide valuable
indications and guide to the experimentalists. As a matter of fact,
DFT is a suitable tool for the analysis of the interactions between
ILs and gas molecules at the nanoscopic level, which allow a
deeper knowledge of the structure–property relationship. Most of
the reported DFT studies only consider CO2.7,12,13,16,17 Though,
some researches leading with SO2 capture have been reported.7,17
There are few recent studies that address utilization of ILs
for gas capture at the molecular level, especially SO2 capture.
Damas et al. have shown a systematic study of acid- and sour-gas
mitigation alternatives (SO2, CO2 and H2S) by using ILs through
DFT simulations, which mainly focuses on imidazolium cation
based ILs.17 In this presented work, we broadened the study that
was conducted by Damas et al. by including other cations such
as piridinium or cholinium cations in combination with anions
such as bis(trifluorosulfonyl)imide, triflate, or tetrafluoroborate
as shown in Table 1 and Fig. 1.
In our opinion, the analysis of those ILs with high efficiency
for SO2 capture through DFT tools should be a good starting
Phys. Chem. Chem. Phys., 2015, 17, 13559--13574 | 13559
View Article Online
Paper
PCCP
Open Access Article. Published on 20 April 2015. Downloaded on 06/02/2017 10:22:27.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
Table 1 The selected family of ionic liquids studied in this work along with their estimated Henry’s Law constants of SO2 (KH) at 303 K predicted using the
COSMO-RS method
No.
Cation
Anion
Labelling
KH 105/Pascal
1
2
3
4
5
6
7
8
9
10
11
12
13
1-Ethyl-3-methylimidazolium
1-Butyl-3-methylimidazolium
1-Hexyl-3-methylimidazolium
1-Methyl-3-octylimidazolium
1-Butylpyridinium
1-Butyl-3-methylpyridinium
1-Butyl-4-methylpyridinium
1-Butyl-3-methylimidazolium
1-Hexyl-3-methylimidazolium
1-Methyl-3-octylimidazolium
1-Butylpyridinium
1-Butyl-3-methylpyridinium
1-Butyl-4-methylpyridinium
Tetrafluoroborate
Tetrafluoroborate
Tetrafluoroborate
Tetrafluoroborate
Tetrafluoroborate
Tetrafluoroborate
Tetrafluoroborate
Hexafluorophosphate
Hexafluorophosphate
Hexafluorophosphate
Hexafluorophosphate
Hexafluorophosphate
Hexafluorophosphate
[EMIm][BF4]
[BMIm][BF4]
[HMIm][BF4]
[OMIm][BF4]
[BPy][BF4]
[B3MPy][BF4]
[B4MPy][BF4]
[BMIm][PF6]
[HMIm][PF6]
[OMIm][PF6]
[BPy] [PF6]
[B3MPy][PF6]
[B4MPy][PF6]
3.69
3.55
3.47
3.35
3.72
2.96
2.96
2.76
2.65
2.61
2.79
2.47
2.46
14
15
16
17
18
19
20
1-Ethyl-3-methylimidazolium
1,3-Dimethylimidazolium
Choline
1-Ethyl-3-methylimidazolium
1-Ethyl-3-methylimidazolium
1-Ethyl-3-methylimidazolium
Ethylammonium
Diethylphosphate
Dimethylphosphate
Dihydrogenphosphate
Ethylsulfate
Hidrogensulfate
Acetate
Nitrate
[EMIm][Et2PO4]
[DMIm][Me2PO4]
[CH][H2PO4]
[EMIm][EtSO4]
[EMIm][HSO4]
[EMIm][Ac]
[EtNH3][NO3]
4.73
4.07
6.29
4.63
6.45
3.85
6.37
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
Triethylsulfonium
1-Ethyl-3-methylimidazolium
1-Methyl-3-propylimidazolium
1,2-Dimethyl-3-propylimidazolium
1-Butyl-3-methylimidazolium
1-Butyl-2,3-dimethylimidazolium
1-Hexyl-3-methylimidazolium
1-Hexadecyl-3-methylimidazolium
1-Allyl-3-methylimidazolium
1-Methyl-1-propylpyrrolidinium
1-Butyl-1-methylpyrrolidinium
1-Methyl-1-propylpiperidinium
1-Butylpyridinium
1-Butyl-3-methylpyridinium
1-Butyl-4-methylpyridinium
Bis[(trifluoromethyl)sulfonyl]imide
Bis[(trifluoromethyl)sulfonyl]imide
Bis[(trifluoromethyl)sulfonyl]imide
Bis[(trifluoromethyl)sulfonyl]imide
Bis[(trifluoromethyl)sulfonyl]imide
Bis[(trifluoromethyl)sulfonyl]imide
Bis[(trifluoromethyl)sulfonyl]imide
Bis[(trifluoromethyl)sulfonyl]imide
Bis[(trifluoromethyl)sulfonyl]imide
Bis[(trifluoromethyl)sulfonyl]imide
Bis[(trifluoromethyl)sulfonyl]imide
Bis[(trifluoromethyl)sulfonyl]imide
Bis[(trifluoromethyl)sulfonyl]imide
Bis[(trifluoromethyl)sulfonyl]imide
Bis[(trifluoromethyl)sulfonyl]imide
[Et3S][NTf2]
[EMIm][NTf2]
[MPIm][NTf2]
[DMPIm][NTf2]
[BMIm][NTf2]
[BDMIm][NTf2]
[HMIm][NTf2]
[HdMIm][NTf2]
[AMIm][NTf2]
[MPPyr][NTf2]
[BMPyr][NTf2]
[MPPipe][NTf2]
[BPy][NTf2]
[B3MPy][NTf2]
[B4MPy][NTf2]
3.52
3.63
3.54
3.19
3.50
3.18
3.50
3.69
3.56
3.39
3.36
3.33
3.49
3.29
3.29
36
37
38
39
40
1-Ethyl-3-methylimidazolium
1-Butyl-3-methylimidazolium
1-Hexyl-3-methylimidazolium
1-Methyl-3-octhylimidazolium
1-Butyl-1-methylpyrrolidinium
Triflate
Triflate
Triflate
Triflate
Triflate
[EMIM][SO3CF3]
[BMIM][SO3CF3]
[HMIM][SO3CF3]
[OMIM][SO3CF3]
[BMPyr][SO3CF3]
4.41
4.20
4.17
4.11
3.57
41
42
43
44
1-Ethyl-3-methylimidazolium
1-Ethyl-3-methylimidazolium
1-Butyl-3-methylimidazolium
1-Butyl-1-methylpyrrolidinium
Thiocianate
Dicyanamide
Dicyanamide
Dicyanamide
[EMIM][SCN]
[EMIM][DCA]
[BMIM][DCA]
[BMPyr][DCA]
4.17
4.30
4.22
2.88
45
46
47
48
49
50
51
52
53
54
55
1-Ethyl-3-methylimidazolium
1-Butyl-3-methylimidazolium
1-Allyl-3-methylimidazolium
1-Ethyl-3-methylimidazolium
1-Butyl-3-methylimidazolium
1,3-Dimethylimidazolium
1-Ethyl-3-methylimidazolium
1-Methyl-3-propylimidazolium
1-Butyl-3-methylimidazolium
1-Hexyl-3-methylimidazolium
1-Allyl-3-methylimidazolium
Chloride
Chloride
Chloride
Bromide
Bromide
Iodide
Iodide
Iodide
Iodide
Iodide
Iodide
[EMIM][Cl]
[BMIM][Cl]
[AMIM][Cl]
[EMIM][Br]
[BMIM][Br]
[DMIm][I]
[EMIm][I]
[MPIm][I]
[BMIm][I]
[HMIm][I]
[AMIm][I]
2.01
3.42
2.87
2.24
3.53
3.39
3.03
3.70
4.07
4.50
4.07
point to shed some light on the main molecular factors related
with efficient SO2 capture. Unfortunately, experimental studies
dealing with SO2 capture by ILs are still scarce and reduced to a
small number of selected ionic liquids. A key parameter in the
selection of an ionic liquid for SO2 capture is gas solubility. It is
13560 | Phys. Chem. Chem. Phys., 2015, 17, 13559--13574
well known that gas solubility in ILs can be predicted based on the
COSMO-RS (Conductor-like Screening Model for Real Solvents)
method.18 The COSMO-RS predicts thermodynamics properties of
solvents on the basis of uni-molecular quantum chemical calculations for the individual molecules, which provides a good
This journal is © the Owner Societies 2015
View Article Online
Paper
Open Access Article. Published on 20 April 2015. Downloaded on 06/02/2017 10:22:27.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
PCCP
Fig. 1
Chemical structure for the ions involved in the selected family of ionic liquids.
prediction of gas solubility data in ILs without prior experimental
knowledge on the compound’s properties.14 Thus, COSMO-RS is
able to carry out fast screening on a huge number of ionic liquids,
reducing the number of candidates for experimental studies,
which also reduces try-and-error attempts and economical and
This journal is © the Owner Societies 2015
temporal cost. Consequently, the COSMO-RS method was firstly
used to carry out a quick screening on a big matrix of ILs. Then,
an in-depth study of those ILs, which are expected to provide high
SO2 solubility according to the COSMO-RS method, from a molecular point of view was done using DFT tools. The combination
Phys. Chem. Chem. Phys., 2015, 17, 13559--13574 | 13561
View Article Online
Paper
PCCP
of first screening to select efficient ILs for SO2 capture using
COSMO-RS analysis along with DFT analysis on the most adequate
ILs have allowed us to obtain information about structure vs.
property relations that control SO2 solubility in ILs, which is crucial
for the rational design of task-specific ILs for SO2 absorption.
Open Access Article. Published on 20 April 2015. Downloaded on 06/02/2017 10:22:27.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
2. Theoretical methodology
2.1.
COSMO-RS method
Four different approaches can be performed to describe ionic
liquids according to the COSMO-RS method. According to Palomar
et al., these approximations are labelled [C + A]GAS, [C + A]COSMO,
[CA]GAS and [CA]COSMO.14 The [C + A] model uses isolated ions to
simulate ionic liquids systems, while ion-paired structures are used
in [CA]. System optimizations can be carried out in gas-phase using
quantum chemistry methods (GAS subscript), or the continuum
solvation COSMO model (COSMO subscript). The [C + A]GAS
approach (i.e. independent ionic structures optimized in the gas
phase) predicts gas solubility data in slightly better agreement with
the experiments. Palomar et al. also concluded that all COSMOS-RS
approaches provide similar good capability to predict Henry’s law
constants for ionic liquid. Nonetheless, the [C + A]GAS model allows
us to perform analysis with a reduced computational time, since
only optimized ion structures in the gas phase are needed, which is
especially useful for screening purposes.
The [C + A]GAS model was employed in this work, which is based
on two main steps: (i) quantum chemical optimization for the
molecular involved species and (ii) COSMO-RS statistical calculations. Firstly, the isolated ions and SO2 were optimized at the B3LYP/
6-311+G(d,p) level using Gaussian 09 (Revision D.01) package,19
which was also instructed to provide the COSMO files. For these
structures, COSMO files were calculated at the BVP86/TZVP/DGA1
theoretical level and used as input in the COSMOthermX program18
to estimate Henry’s law constants. The COSMO-RS model parameterization used for all calculations was BPTZVPC21-0111.
In this work, Henry’s law constants (KH) for SO2 were selected
as a measure of absorbing capability. Henry’s constants are
directly calculated by COSMOthermX code. The details of theory
of COSMO-RS can be found in the original work of Klamt et al.18
Briefly, Henry’s law constants can be defined as the ratio
between the liquid phase concentration of SO2 and its partial
vapour pressure in the gas phase:
S
KH = Pi/xi = gN
i Pi
(1)
where Pi and xi are the partial vapour pressure of a compound i
(SO2 in our study) in the gas phase and its molar fraction in the
liquid. gN
i is the activity coefficient of the compound at infinite
dilution, and PSi is the saturated pure compound vapor pressure
of the gas. Those parameters are directly provided by the
COSMOthermX code.
2.2.
DFT simulations
Systems composed by one isolated molecule (i.e. isolated ions
and SO2) up to the system composed by both ions and SO2 were
optimized. Optimized minima were checked through their
13562 | Phys. Chem. Chem. Phys., 2015, 17, 13559--13574
vibrational frequencies. For those simulations wherein two or
more molecules are present, different starting points were
employed in order to study different relative dispositions,
focusing our attention on the disposition of minimal energy.
All these calculations were carried out using a B3LYP-D2
functional. B3LYP20 has been selected since it has been proven
to show appreciable performance over a previously studied
wide range of systems,21 while dispersion corrections (D2) are
adequate since we dealt with systems with dispersive interactions such as hydrogen bonds.22 In addition, other works
dealing with the performance of dispersion corrected functionals to study ionic liquid concluded that dispersion correction could significantly decrease mean absolute deviations for
binding energies up to 10.0 kJ mol1 or lower in comparison
with the MP2 method.23 All atomic elements, except iodine,
were described with the standard Pople basis set 6-311+G(d,p).
For iodine, a small core Stuttgart–Dresden–Bonn effective core
potential was used (SDB-cc-pVTZ).24 Interaction energies (BE)
related with SO2 capture were computed as the energy difference
between the complex and the sum of the energy of each
component. For example, BE for IL SO2 was calculated as:
BE = EIL–SO2 (Ecat + Eani + ESO2)
(2)
Binding energies were also estimated by considering the IL as
a whole (BE 0 ), i.e., the binding energy due to the interaction
between the IL and the gas molecule:
BE 0 = EIL–SO2 (EIL + ESO2)
(3)
where EIL–SO2, Ecat, Eani, EIL and ESO2 stand for the energies of
IL SO2, cation, anion, IL and SO2, respectively. For those
systems composed of two or more molecules, computed energies
were corrected according to the counterpoise method to avoid
basis set supper position error (BSSE).25
It has been shown that there is a specific charge transfer
interaction between SO2 and the ions.26 There are different
methods to calculate charge distributions, such as the Mulliken
method,27 whose basis set dependence is well known.28 ChelpG
scheme29 has demonstrated its suitability for ILs.12,30 Thus,
atomic charges were also computed according to both ChelpG
and Mulliken schemes. Intermolecular interactions where analyzed
in the framework of Bader’s theory (Atoms in Molecules, AIM).31 In
this context, intermolecular interactions are characterized through
critical points (CP). Although four kind of critical points were
obtained, we focused on bond critical points (BCP), which raises
the criteria for considering the presence of intermolecular interactions.31 AIM analysis was carried out with the MultiWFN
code.32 All the above-mentioned calculations were carried out
with Gaussian 09 (Revision D.01) package.19
3. Results and discussion
3.1.
COSMO-RS analysis: selection of the optimal IL family
As said, the first step in our study was the selection of an
optimal family of ILs with high SO2 solubility. The SO2 absorption capacities were evaluated in terms of Henry’s law constants
This journal is © the Owner Societies 2015
View Article Online
Open Access Article. Published on 20 April 2015. Downloaded on 06/02/2017 10:22:27.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
PCCP
Paper
(KH) predicted according to the COSMO-RS method. COSMO-RS
is a predictive method for thermodynamic equilibrium of
fluids, which uses a statistical thermodynamic approach based
on the results of uni-molecular quantum chemical calculations.
The efficiency of COSMO-RS to predict the solubility behaviour
of different solutes in ILs was evaluated by comparing both the
experimental and computed (according to the COSMO-RS
method) Henry’s constants.14,18,33,34 Although some publications have reported that COSMO-RS systematically overestimates
the Henry’s constants, it provides a reasonable linear fit between
the calculated and experimental values.14,34
In this work, COSMO-RS approach has been used to perform
a fast screening on SO2 solubility in ILs. Although several
properties, such as s-surfaces, screening charge density,
s-profiles, and histograms of screening charge can be computed with COSMO-RS, we have focused on Henry’s law constants for SO2 as a measure of absorbing capability. For this, KH
(at 303 K) was estimated for a matrix of C7600 ILs formed
through a combination of cations based on imidazoluim,
piperidinium, choline, ammonium cations paired with anions
such as halogens, phosphates, tetrafluoroborate, dicyanamide
or bis(trifluorosulfonyl)imide (see Table S1, ESI†). In addition
to low Henry’s law constants, only those ILs with an adequate
viscosity profile for industrial applications as suitable ILs for
SO2 capture were considered. Thus, a set of 55 ILs (see Table 1
and Fig. 1) was selected for a deeper DFT analysis. Table 1 and
Fig. 2 gather the computed Henry’s law constants of the
selected ionic liquids. All selected ILs yield KH within the range
of 2.5 105 to 6.5 105 Pascal at 303 K. These values are
smaller (which means higher solubility) than those reported
by González-Miquel et al. (of around 30 105 Pascal–60 105 Pascal) for CO2 absorption.35 Then, high efficiency for SO2
capture can be expected for selected ILs. Note that most of the
selected ILs are based on cations such as imidazolium, pyridinium or piperazinium and anions such as [BF4], [PF6],
[NTf2] triflate or halides. Then, the combination of these
anions would be adequate to design ILs for SO2 capture with
high efficiencies.
Fig. 2
3.2.
DFT analysis
As a first approximation, SO2 capture at the molecule level
could be related with the strength of the interactions between
the ions and the SO2 molecule. In this work, the interaction
strength has been mainly analyzed based on binding energies
(BE). Prior to analysis of SO2 capture by selected ILs, ion SO2
and ionic pairs were also briefly assessed. Such information
could be useful to rationalize the behavior of IL SO2 systems.
3.2.1. Ion SO2 systems. Fig. 3 shows computed binding
energies (|BE|) for anion SO2 interactions. In general, the
selected cations provide similar |BE|, whose values lie between
31.70 kJ mol1 ([BMPyr]+) and 42.92 kJ mol1 ([CH]+), except
[EtNH3]+ which yields the largest cation SO2 values, (|BE| =
58.60 kJ mol1). In concordance with Damas’s work, the binding energy for the imizadolium family decreases upon alkyl side
chain elongation. In fact, from [EMIM]+ to [HMIM]+, BE varies
by only 1.83 kJ mol1. However, larger alkyl side chains such as
[OMIM]+ and [HdMIM]+ lead to a slight increase in BE upon
chain elongation. Anion SO2 binding energies are, in general,
larger than cation SO2 binding energies, with values varying
between 41.91 kJ mol1 ([NTf2]) and 123.37 kJ mol1
([H2PO4]). Some ions can be classified according to their
chemical structure (such as those based on phosphate or
sulfate anions). Thus, |BE| of those ones based on dialkyl
phosphate slightly decreases (C4.00 kJ mol1) upon alkyl
chain elongation. The alkyl chain absence in [H2PO4] leads
to |BE| values of 29.12 kJ mol1 which is greater than that of
[Et2PO4]. Similar patterns are noted for sulfate-based ions,
wherein the presence of an ethyl chain leads to a diminution of
16.48 kJ mol1. As concerns as halides, |BE| = 52.18 kJ mol1
(in average). In order to compare BE values with experimental
data, IL 22 ([EMIM][NTf2]) has been selected as its CO2
capture performance has been demonstrated experimentally.11
According to eqn (2), CO2 capture by IL 22 yields |BE| 0 =
36.58 kJ mol1. This energy could be considered as a low limit,
from which higher |BE| would be adequate to provide high
SO2 affinities.
Inverse of Henry’s Law constants of SO2 in ILs (1/KH) at 303 K predicted using the COSMO-RS method.
This journal is © the Owner Societies 2015
Phys. Chem. Chem. Phys., 2015, 17, 13559--13574 | 13563
View Article Online
Open Access Article. Published on 20 April 2015. Downloaded on 06/02/2017 10:22:27.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
Paper
PCCP
Fig. 3 Computed binding energies (in absolute value, |BE|) of cation SO2
(up) and anion SO2 (bottom) systems.
Fig. 4 Charge transfer of cation SO2 (up) and anion SO2 (bottom)
systems.
It has been proven that there is a charge transfer interaction
between SO2 and the anion motif of ILs. This charge transfer
interaction is proportional to the anion basicity and plays an
important role on the gas adsorption capacity.26 Fig. 4 collects
the charge transfers between the cation/anion and SO2 molecule.
For cation (anion) SO2 systems, the total charge over the SO2
molecule takes positive (negatives) values, which means that
charge is transferred from the SO2 up to the cation (from the
anion up to the SO2). Broadly, charge populations according to
the Mulliken scheme are smaller than those computed using the
ChelpG model. According to ChelpG (Mulliken) atomic charges,
charge transfer between cations and the SO2 molecule is, on
average, 0.05 (0.05) electrons. Thus, van der Waals interactions
are one of the main contribution to the |BE| for cation–SO2
systems, which is in concordance with lower |BE| values than
anion–SO2 systems. Now, the total charge over SO2 molecule is
0.23(0.21) electrons for anion–SO2 systems. These higher values
are in concordance with greater anion appetency to interact with
the SO2 molecule due to a charge transfer interactions. Fig. 5
shows the relationship between binding energies and charge
transfer of anion SO2 systems (a similar pattern has not been
found for cation SO2 systems), which follows a linear behavior
for most anions.
3.2.2. Ionic liquids. Fig. 6 gathers computed |BE| of the
isolated ionic pair and the charge transfer (CT) between ions
according to the ChelpG scheme. Most ILs yield |BE| between
318.99 kJ mol1 (IL 21) and 492.70 kJ mol1 (IL 20), while
ILs 25, 29, 31, 33 and 35 provide the smallest values, around
173.09 kJ mol1. As known, the columbic attraction between
opposite charges is the main force between both ions forming
the ionic liquid. Even though, other intermolecular forces can
also be present. Both the charge transfer and BE follow similar
patterns (Fig. 6), i.e., the columbic interaction between both
positive and negative charges is one of the main contributions
to the binding energy. ILs with the smallest |BE|, i.e. IL 25, 29,
31, 33 and 35, are those wherein high charge transfer does not
provide high binding energies, which points out that other
interactions (such as hydrogen bonds) also represent an important contribution (intermolecular interactions between ions
are below described for some ILs). ILs based on halide anions
(45–55) show increasing CT with the halide electronegativity.
Those effects are stronger from chloride to bromide halides.
CTs and binding energies depend on both the cation and anion
nature as well. For instance, those ILs based on imidazol
cations and [NTF2] anions (except ILs 25 and 29) yield similar
|BE| (C339.0 kJ mol1).
13564 | Phys. Chem. Chem. Phys., 2015, 17, 13559--13574
This journal is © the Owner Societies 2015
View Article Online
Open Access Article. Published on 20 April 2015. Downloaded on 06/02/2017 10:22:27.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
PCCP
Fig. 5
Paper
Binding energies (|BE|) vs. charge transfer of anion SO2 systems.
3.2.3. SO2 capture by ionic liquids. Binding energies of
IL SO2 systems have been used as a measurement of the
interaction strength between selected ILs and SO2 molecule.
Fig. 7 collects |BE| (according eqn (2)) of IL SO2 systems,
which has been decomposed as a sum of the ionic pair,
cation SO2 and anion SO2 contributions. Thus, using the
optimized IL SO2 geometries, contributions from cation–
anion, cation SO2 and anion SO2 have been also calculated.
BE energies were also estimated taking into account the ILs as a
whole (eqn (2), |BE 0 |). All these quantities are also provided in
Fig. 7. The largest contribution to the binding energy comes
from the interaction between both ions. For an easier comparison, this contribution has been also represented in Fig. 6.
For most ILs, the SO2 molecule only induces a scarce weakening
on the interaction between ions (lower |BE|). However, ILs with
the lowest |BE| in the absence of SO2 (ILs 25, 29, 31, 33 and 35,
see Fig. 6) are those wherein the SO2 molecule steers to a
strengthening on the interaction between ions. This is due to
the phenomena that the new arrangement between ions of
SO2 improves the interaction between both ions and their
interactions with the gas molecule, which is described in
detail below.
Regarding ion SO2 contributions, cation SO2 one is, in
general, much lower than anion SO2 contributions. Even if,
the behavior of both ion SO2 contributions and its relationship with |BE| 0 depends on the analyzed IL. For instance,
anion SO2 contributions present similar values to |BE 0 | for
ILs 1–6 (based on tetrafluoroborate anion) and 45–55 (based on
halides), i.e., anion SO2 interactions stand for the main
contribution to the total binding energies of these IL–SO2
systems. Hence, for those ILs based on [BF4] (1–6) and halide
(45–55) anions, the SO2 adsorption process is mainly governed
by the anion. For ILs based on triflate, thiocianathe or dicyanamide
(ILs 36–44), the sum of both ion SO2 contribution yields similar
values of |BE0 |. In consequence, the SO2 capture using ILs 36–44
would be guided by both ions. Based on average values, binding
energies of cation/anion–SO2 systems (Fig. 3) yield values
C37.09 kJ mol1/72.37 kJ mol1. However, cation/anion SO2
contributions to the binding energy (Fig. 7) are of around
13.94 kJ mol1/52.03 kJ mol1. For both ions, interaction
energies reduce C21.9 kJ mol1 due to the presence of the
paired ion. Ions became less negative, since they transfer
charge up to both the cation and SO2 molecule. However, both
ions strongly interact between them, hindering cation/
anion SO2 interactions. Afresh, this general trend depends
on the selected family. For example, for [BF4]/[Cl]/[Br]/[I]
anion |BE| = 55.45 kJ mol1/54.64 kJ mol1/50.07 kJ mol1/
51.83 kJ mol1, while anion–SO2 contributions to the total |BE 0 |
for ILs 1–6 (which are those based on tetrafluoroborate anion)
are C69.28 kJ mol1, and C90.07 kJ mol1 for those ILs based
on halides (45–55). As seen above, anion SO2 interactions are
mainly ruled by the anion SO2. Both factors point out that the
CT between both ions would increase anion basicity, as well as
its interaction strength with the SO2 molecule. Bearing in mind
Fig. 6 Computed binding energies (in absolute value, |BE|) of ionic pairs (black line), along charge transfer computed according to the ChelpG scheme
(blue bar). Binding energies of ionic pairs using their geometries in the presence of SO2 are also collected (green line).
This journal is © the Owner Societies 2015
Phys. Chem. Chem. Phys., 2015, 17, 13559--13574 | 13565
View Article Online
Open Access Article. Published on 20 April 2015. Downloaded on 06/02/2017 10:22:27.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
Paper
PCCP
Fig. 7 Computed binding energies (in absolute value, |BE|) of ILs SO2 (orange line), along anion SO2 contributions (red bar), cation SO2 (blue bar)
and cation anion (green bar). Computed binding energies of ILs SO2 considering the IL as a whole are also collected (black line).
In short, anion SO2 interactions play an important role
in SO2 capture by ILs. When both ions are considered,
anion SO2 strengths will be affected by cation–anion interactions. We have defined the binding energies of IL–SO2
systems (BE, according eqn (2)) as a function of the BE of
ion–SO2 systems (Section 3.2.1. and Fig. 3) and ionic pairs
(Section 3.2.2. and Fig. 6):
BEIL–SO2 = (aBECAT–SO2) x + (bBEANI–SO2) y + (cBEIL) z
Fig. 8 Computed charge over SO2 (blue), cation (green) and anion (red)
according to the ChelpG scheme. Dotted lines correspond to ion charge
for isolated ILs.
a value of around 36.0 kJ mol1 (estimated for CO2 capture by
[EMIM][Tf2N] IL) as a low limit, almost ILs yield larger values,
|BE 0 | C 45.0 kJ mol1. According to these raised values, an
efficient SO2 capture can be expected. Once more, ILs 25, 29, 31,
33 and 35 do not follow the general trend, since their binding
energies despising ionic contribution (|BE 0 |) are much larger
than the sum of both ion SO2 contributions.
Total charges over both ions and the SO2 molecule are
displayed in Fig. 8. The gas usually gets a negative charge,
i.e., there is a charge transfer for the anion up to the SO2
molecule. Charge populations over both ions for ILs in the
absence of SO2 are also included in Fig. 8. According to the
ChelpG scheme, cationic charges slight vary due to the SO2
molecule, while anionic charges suffer drastic lessening due to
the charge transfer up to the SO2 molecule.
13566 | Phys. Chem. Chem. Phys., 2015, 17, 13559--13574
(4)
where BEIL–SO2, BECAT–SO2, BEANI–SO2, BEIL are the binding
energies of the IL–SO2, cation–SO2, anion–SO2 and anion–
cation systems, respectively, while a, b, c, x, y and z are
adjustable parameters. Fig. 9a plots the results of a statistical
analysis after expressing BEIL–SO2 according to eqn (4). Fig. 9a
gathers the data collected for the whole set of ILs. Most of them
yield a linear behavior between BEIL–SO2 estimated from the
IL–SO2 optimized systems (BEIL–SO2,DFT) and those ones after
the fit of eqn (4) (BEIL–SO2,Statistical). Hence, the total binding
energy of IL–SO2 systems, which takes into account both anion–
cation and anion–SO2 interactions, could be directly obtained
through the optimization of ion SO2 systems and ILs. The fit
yields R2 = 0.6772 and medium deviation (MD) = 3.20 kJ mol1,
which could be considered an acceptable value despite the
variety in the chemical structure of the selected ionic liquid.
The largest errors correspond to ILs 25, 29, 31, 33 and 35
(|BEIL–SO2,Statistical| C 360.0 kJ mol). As seen above, those ILs
suffered and strengthening of the interaction between ions due
to the presence of the SO2 molecule occurred. According to
eqn (4), no important differences on binding energies for ILs
are expected upon SO2 presence. On the other hand, IL 20
(|BEIL–SO2,Statistical| = 578.34 kJ mol) is based on [EtNH3]+ cation.
[EtNH3]+–SO2 provided the highest binding energy among all
studied cations. According with a parameter (a = 1.28 1016),
contribution from BECAT–SO2 is close to zero. Hence, the above
expression is only applicable to those ILs wherein the anion
plays the main role on SO2 capture and for those ILs which do
This journal is © the Owner Societies 2015
View Article Online
Open Access Article. Published on 20 April 2015. Downloaded on 06/02/2017 10:22:27.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
PCCP
Fig. 9
Paper
Results from the fit of BEIL–SO2 according to eqn (4) and (5).
not suffer important geometrical arrangements in the presence
of the gas molecule. Note that the z parameter is close to one.
As a result, we defined eqn (4) based only on BEANI–SO2 and BEIL.
BEIL–SO2,Statistical as follows:
BEIL–SO2,Statistical = b(BEANI–SO2) y + cBEIL
(5)
The fit was repeated despising ILs 20, 25, 29, 31, 33 and 35.
As seen in Fig. 9b, there is a notable improvement in the fit
performance with R2 = 0.7887 and MD = 2.55 kJ mol1. It could
be concluded that SO2 capture by ILs is mainly governed by ILs,
while interactions between ions are also an important parameters. Since BE between ions is much higher than anion–SO2 ones,
BEIL grants the most important contribution to BEIL–SO2,Statistical.
Then, for those ILs with similar BEIL, the efficiency in SO2 capture
will be ruled by the anion. In addition, eqn (5) allows estimating
BEIL–SO2 only through the optimization of anion–SO2 and cation–
anion systems, which can be considered a useful insight into
the rational design of ILs for SO2 capture.
3.2.4. Representative ionic liquids for SO2 capture. Up to
now, properties for IL SO2 interactions have been analyzed
for the whole family of selected ILs based on binding energies.
As seen, the SO2 absorption capacity is often governed by
anion SO2 interactions, although cations have also an important role. Even though, cation–SO2 contributions to the total
|BE 0 | are always lower than cation–SO2 binding energies, while
this general trend was not found for anion SO2 contributions.
For instance, anion SO2 contributions to the total |BE 0 | for
ILs (1–4, which are based on imidazolium cations paired with
tetrafluoroborate anion) are higher than binding energies for
anion–[BF4] systems, while the opposite trend was noted for
ILs 22–29 (also based on imidazol derived cations, but paired
with the [NTf2] anion). In addition, a statistical analysis has
shown that BE of IL–SO2 systems mainly depends on anion–SO2
and cation–anion interactions. The diversity in the nature of
both ions forming the family of studied ILs hinders the search
of structure–property relationships. Therefore the IL family has
been divided into six sets (labelled as I–VII, see Fig. 2 and 7),
wherein ILs within the same sets have similar features regarding the chemical structure of their ions. For each one, the most
This journal is © the Owner Societies 2015
representative ILs have been selected, whose intermolecular
interactions where analyzed within the context of the AIM
theory to obtain some information on the SO2 capture mechanism
at the nanoscopic level.
Set I (ILs 1–13) includes ILs based on imidazolium ([Im]+) or
pyridinium ([Py]+) cations paired with [BF4] or [PF6] anions.
ILs based on imidazolium and [BF4] (ILs 1–4) yields similar
KH C 3.6 105 Pascal and |BE 0 | C 49.65 kJ mol1. [Im][PF6]
based ILs (8–10) render smaller KH (C2.7 105 Pascal);
however this improvement in KH is not observed on |BE 0 |
(C45.19 kJ mol1). For pyridinium based ILs (5–7 and 11–13)
The replacement of [Im]+ by [Py]+ does not lead to important
changes on KH and |BE 0 |. Though, the alkyl chain length in the
cation, as well as the presence of [BF4] or [PF6] anions have
an effect of thermophysical properties such as viscosity or
density.30,36 ILs included in set I would provide similar SO2
capture efficiency (based on KH and |BE 0 | values). As a matter of
fact, several papers highlight the effect on macroscopical
properties as a function of the selected ions elsewhere.30,36,37
In order to discuss the effects on different ions at the molecular level, besides previously described parameters, the interaction mechanisms of [BMIm][BF4] (IL 2), [BMIm][PF6] (IL 8),
[B4MPy][BF4] (IL 7) and [B4MPy][PF6] (IL 13) have been deeply
analyzed as representative compounds of this set. Intermolecular interactions were localized and featured through the AIM
theory (we have focused on electronic density values, r, for the
main intermolecular interactions). Fig. 10 plots their optimized
structures in the presence of the SO2 molecule (optimized
geometries for isolated ILs are not represented since the
presence of SO2 does not carry out important changes on the
relative disposition between ions), whereas bond length and
AIM features of intermolecular interactions are reported in
Table 2. In the absence of the SO2 molecule, several anion–
cation interactions are established. The main interactions are
formed between F and H in position 2 of the imidazolium/
pyridinium ring, whose d (intermolecular distance) and r are
C2.240 and 0.0140 a.u., respectively. In this sense, it is well
known that the main interaction in imidazilium based ILs is
carried out through the H atom in position 2.17 The presence
of the SO2 molecule leads to an intermolecular distance
Phys. Chem. Chem. Phys., 2015, 17, 13559--13574 | 13567
View Article Online
Open Access Article. Published on 20 April 2015. Downloaded on 06/02/2017 10:22:27.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
Paper
PCCP
Fig. 10 Optimized geometries of [BMIm][BF4] (2), [B4MPy][BF4] (7), [BMIm][PF6] (8) and [B4MPy][PF6] (13) in the presence of the SO2 molecule. Main
intermolecular interactions are also displayed. Atom colour code: C (gray), oxygen (red) sulphur (yellow), hydrogen (white), nitrogen (blue), boron (pink),
phosphorous (orange) and fluorine (light blue). See Table 2 for a more detailed description on intermolecular interactions.
elongation and electronic density decrease, in concordance
with lower |BE| of ILs using their geometries in the presence
of SO2. As seen below, this effect is also noted for almost all ILs
under study. Anion–SO2 interactions are mainly characterized
by a BCP between F and S, labeled as d6, d15, d24 and d33 for ILs
2, 7, 8 and 13, respectively, whose r are 0.0273 a.u., 0.0189 a.u.,
0.0135 a.u., 0.0092 a.u., respectively. Similar patterns are noted
for anion–SO2 contribution to the total binging energies
(Fig. 7). Cation–SO2 interactions take place through O (SO2)
and H (cation). These H are mainly located on the alkyl side
chain. For ILs based on [B3MPy]+/[B4MPy]+ based ILs (6/7 and
12/13), the presence of methyl chain in position 3/4 brings
slight improvement on KH and BE with respect to [BPy]+. This
methyl group in position 3/4 allows an additional intermolecular interaction (e.g., d13 for IL 7) with SO2, which is absent
for [BPy]+.
ILs based on phosphate, sulfate, acetate or nitrate anions are
located in set II. Most of them are also based on [EMIm]+
cation. Interaction energies of anion SO2 systems (see Fig. 3);
[Et2PO4] (99.24 kJ mol1), [EtSO4] (65.75 kJ mol1), [Ac]
(120.47 kJ mol1) and [NO3] (85.06 kJ mol1), are larger than
those estimated for [EMIm]+ (33.77 kJ mol1) and [EtNH3]+
(58.60 kJ mol1) cations. Analogous behaviour is noted for both
ion SO2 contribution to the binding energy, i.e., anion SO2 4
cation SO2. Nevertheless, the sum of both contributions
is higher than |BE 0 | (see Fig. 7). In concordance with |BE|
computed for cation/anion SO2, anion SO2 contribution
to the total BE is larger. Within this set we have focused on
13568 | Phys. Chem. Chem. Phys., 2015, 17, 13559--13574
[EMIm][Et2PO4] (14), [EMIm][EtSO4] (17) and [EMIm][Ac] (19)
ILs. A detailed analysis of [CH][H2PO4] at the molecular level and
their application for SO2 capture will be studied in a separate
work in the future. The structures of [EMIm][Et2PO4] (14),
[EMIm][EtSO4] (17) and [EMIm][Ac] (19) in the presence and
absence of SO2 are reported in Fig. 11. [EtNH3][NO3] (20) IL has
been also selected to obtain some insight up to the behaviour of
this IL. In the absence of the SO2 molecule, the main interaction
between imidazolim cation and the corresponding anion is
carried out by a hydrogen bond between the O atom (anion)
and H in position 2 of the imidazolium ring (labelled d1, d8 and
d16 for ILs 14, 17 and 19 respectively). Again, the presence of the
SO2 molecule brings a diminution of the interaction between
both ions. As seen in Fig. 7 for IL 17, contribution from
anion SO2 interaction to the |BE| is larger than cation SO2
interaction, which agrees with larger r values for d12 regarding to
interaction between cation and SO2, i.e., d13 and d14 (similar
behaviour can be drawn for ILs 14 and IL19). The SO2 molecule
interacts with the anion through an intermolecular bond
between the S and one oxygen atom located in the anion.
[EtNH3][NO3] (20) presents the highest charge transfer and |BE|
between ions in the absence of SO2 (see Fig. 6). As seen in Fig. 11,
there is a proton transfer between ions. In fact, the distance
between [NO3] and H (d23) is 1.045 Å, while the distance
between N and H (d24) is 1.595 Å. ChelpG charges have shown
that such O has an atomic charge of 0.58 (larger than the
0.46 e over the other O), while the positive charge over this H
is 0.39 (charge over remaining H linked to N is of around 0.26 e).
This journal is © the Owner Societies 2015
View Article Online
PCCP
Paper
Table 2 Intermolecular distances (d) along the electronic density values
(r) of [BMIm][BF4] (2), [B4MPy][BF4] (7), [BMIm][PF6] (8) and [B4MPy][PF6]
(13) ionic liquids. See Fig. 10 for labeling
IL SO2
IL
Open Access Article. Published on 20 April 2015. Downloaded on 06/02/2017 10:22:27.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
d/Å
2 – [BMIm][BF4]
2.233
d1
2.106
d2
2.502
d3
2.873
d4
2.222
d5
d6
d7
d8
7 – [B4MPy][BF4]
2.298
d9a
2.119
d10
2.671
d11a
2.322
d12
d13
2.289
d14
d15
d16
d17
8 – [BMIm][PF6]
2.406
d18
2.320
d19
2.413
d20
2.479
d21
2.698
d22b
2.488
d23
d24
d25
d26
d27
13 – [B4MPy][PF6]
2.328
d28
2.124
d29
2.313
d30
a
2.588
d31
d32
d33
d34
d35
d36
r/a.u.
d/Å
r/a.u.
0.0143
0.0178
0.0082
0.0101
0.0130
2.788
2.457
2.502
2.892
2.447
2.531
2.518
2.118
0.0076
0.0121
0.0091
0.0102
0.0100
0.0273
0.0087
0.0170
0.0129
0.0173
0.0117
0.0097
2.910
2.549
2.484
2.870
2.479
2.706
2.716
2.496
2.429
0.0097
0.0148
0.0081
0.0105
0.0077
0.0188
0.0089
0.0082
0.0097
0.0113
0.0138
0.0107
0.0100
0.0142
0.0097
2.421
2.607
2.701
2.315
2.803
2.648
2.652
2.209
2.535
2.548
0.0116
0.0101
0.0067
0.0118
0.0094
0.0206
0.0135
0.0084
0.0071
0.073
0.0116
0.0167
0.0113
0.0114
2.363
2.078
2.481
2.446
2.698
2.648
2.955
3.218
2.718
0.0128
0.0181
0.0145
0.0095
0.0057
0.0092
0.0065
0.0059
0.0058
0.0120
a
For isolated IL, this interaction take places between F and H in
position 2. b For isolated IL, this interaction takes places between F
and C in position 2.
This effect is not observed in the presence of the SO2 molecule.
The adsorption of SO2 by IL 20 is carried out by a strong
interaction between SO2 and the anion (d25), while there are dual
interactions between SO2 and the cation (d26 and d27, being the
latter the weakness). Once more, a larger electronic density for
d35 (respect to d26) agrees with the greater contribution from
SO2 interaction to |BE0 |.
ILs based on the [NTf2] anion (set III) are the largest group,
whose KH C 3.4 105 Pascal and |BE 0 | C 38.45 kJ mol1
(despising ILs 25, 29, 31, 33 and 35). For ILs 25, 29, 31, 33 and 35,
|BE 0 | is much larger than sum of both ion SO2 contributions.
Furthermore, ILs 25, 29, 31, 33 and 35 are the only ones whose
interactions between ions are strengthened in the presence of
This journal is © the Owner Societies 2015
Fig. 11 Optimized geometries of [EMIm][Et2PO4] (14), [EMIm][EtSO4] (17),
[EMIm][Ac] (19) and [EtNH3][NO3] (20). Main intermolecular interactions are
also displayed. Atom colour code: C (gray), oxygen (red) sulphur (yellow),
hydrogen (white), nitrogen (blue)and phosphorous (orange). See Table 3
for a more detailed description on intermolecular interactions.
the SO2 molecule (see Fig. 6). The SO2 brings a rearrangement
between ions which improves their mutual interaction and also
their interactions with SO2. To obtain information about this
fact, we have focused on IL [BMIm][NTf2]. Optimized geometries
as well as the main results from intermolecular interaction
Phys. Chem. Chem. Phys., 2015, 17, 13559--13574 | 13569
View Article Online
Paper
PCCP
Table 3 Intermolecular distances (d) along the electronic density values
(r) of [EMIm][Et2PO4] (14), [EMIm][EtSO4] (17), [EMIm][Ac] (19) and [EtNH3][NO3]
(20). See Fig. 11 for labeling
IL SO2
IL
Open Access Article. Published on 20 April 2015. Downloaded on 06/02/2017 10:22:27.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
d/Å
14 – [EMIm][Et2PO4]
1.756
d1
2.304
d2
1.962
d3
d4
d5
d6
d7
17 – [EMIm][EtSO4]
2.055
d8
2.510
d9
2.417
d10
2.184
d11
d12
d13
d14
d15
19 – [EMIm][Ac]
d16
1.654
2.352
d17
2.002
d18
d19
d20
d21
d22
20 – [EMIm][NO3]
1.596
d23
1.045
d24
d25
d26
d27
r/a.u.
d/Å
r/a.u.
0.0299
0.0120
0.0253
1.793
2.780
2.119
2.238
2.510
2.578
2.242
0.0369
0.0194
0.0064
0.0575
0.0088
0.0087
0.0144
0.0231
0.0095
0.0103
0.0159
2.052
2.500
2.513
2.555
2.243
2.419
2.444
2.576
0.0225
0.0096
0.0087
0.0082
0.0396
0.0109
0.0081
0.0059
0.0278
0.0109
0.0240
2.110
2.491
2.427
2.189
2.667
3.665
2.583
0.0205
0.0097
0.0104
0.0658
0.0077
0.0128
0.0082
1.639
2.625
2.350
1.820
2.620
0.0564
0.0078
0.0454
0.0328
0.0074
0.0072
Fig. 12 Optimized geometries of [BMIm][NTf2] (25), [BMPyr][NTf2] (31) and
[B4MPy][NTf2] (35). Main intermolecular interactions are also displayed.
Atom colour code: C (gray), oxygen (red) sulphur (yellow), hydrogen
(white), nitrogen (blue) and phosphorous (orange). See Table 4 for a more
detailed description on intermolecular interactions.
Table 4 Intermolecular distances (d) along the electronic density values
(r) of [BMIm][NTf2] (25). See Fig. 12 for labeling
IL SO2
IL
analysis are collected in Fig. 12 and Table 4. [BMIm][NTf2] ILs
yields five intermolecular interactions (d1–d5) between both ions,
wherein the one between the N (anion) and the H (cation) is
position 2 is the strongest one. Although the same interactions
between both ions are found in the presence of the SO2 molecule, all of them suffer an elongation/decrease on intermolecular
distances/electronic density values. SO2 molecule is able to for
two bonds with the anion, i.e., d6 (S O) and d7 (S F), being the
latter much weaker than S O interaction. Further, two O H
bonds (d8 and d9) are noted between SO2 and cation molecules.
Although SO2 causes a weakening of the interaction between
ions (based on electronic density values), it also allows the
formation of a cage, with their corresponding cage critical points
(CCP). Concretely, two cage critical points (represented as purple
points along the yz view) are found, whose electronic density is
0.0027 a.u. and 0.0018 a.u. The presence of both CCP points out
to a charge delocalization process between different motifs.
Results described for this IL could be extrapolated to ILs 29,
31, 33 and 35, i.e., larger |BE 0 | values and stronger interaction
between ions are due to the charge delocalization process. This
charge delocalization brings an increase on inter ionic interaction (with respect to isolated IL), and |BE 0 | is higher than the
sum of both ion SO2 contributions. Although, CCPs are also
13570 | Phys. Chem. Chem. Phys., 2015, 17, 13559--13574
d/Å
25 – [BMIm][NTf2]
1.952
d1
2.186
d2
2.290
d3
2.564
d4
2.553
d5
d6
d7
d8
d9
r/a.u.
d/Å
r/a.u.
0.0303
0.0150
0.0130
0.0063
0.0024
2.020
2.419
2.352
2.594
2.623
2.683
3.208
2.739
2.479
0.0266
0.0104
0.0115
0.0058
0.0058
0.0218
0.0062
0.0068
0.0099
found for other ILs, they own much lower electronic density
values.
ILs based on the triflate anion ([SO3CF3]) are within set IV
(IL 36–40). Those ones also based on imidazolium cations
(36–39) provide KH C 4.22 105 Pascal and |BE 0 | C
60.30 kJ mol1, which is due to the sum of both ion SO2
contributions. [BMPyr][SO3CF3] (IL40) yields KH = 3.57 105
Pascal and |BE 0 | = 51.97 kJ mol1, mainly due to the
anion SO2 contribution. Larger anion SO2 contributions
(Fig. 7) to the binding energy mimic the previously reported
compound for ion SO2 binding energies (Fig. 3). Fig. 13 and
Table 5 gather optimized geometries and intermolecular
This journal is © the Owner Societies 2015
View Article Online
Open Access Article. Published on 20 April 2015. Downloaded on 06/02/2017 10:22:27.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
PCCP
Paper
Fig. 13 Optimized geometries of [BMIm][SO3CF3] (37) in the presence of
SO2 (similar results are obtained for isolated IL). Main intermolecular
interactions are also displayed. Atom colour code: C (gray), oxygen (red)
sulphur (yellow), hydrogen (white), nitrogen (blue) and fluorine (light blue).
See Table 5 for a more detailed description on intermolecular interactions.
Table 5 Intermolecular distances (d) along the electronic density values
(r) of [BMIm][SO3CF3] (37). See Fig. 13 for labeling
IL SO2
IL
d/Å
37 – [BMIm][SO3CF3]
2.025
d1
2.280
d2
2.520
d3
2.594
d4
d5
d6
d7
r/a.u.
d/Å
r/a.u.
0.0239
0.0130
0.0081
0.0088
2.184
2.626
2.439
2.544
2.476
2.464
2.451
0.0070
0.0109
0.0089
0.0351
0.0096
0.0107
0.0070
parameters for [BMIm][SO3CF3] (37). Results obtained for this
IL could be extrapolated for the whole set IV. The main
interaction between both ions takes places through O corresponding to the anion and H in position 2 located in the cation
(d1), whose r and distances are more affected by the SO2
molecule, which causes its weakening. However, the remaining
interactions are slightly affected by the gas molecule. Thus,
binding energy for IL 37 is very similar to contribution from
inter ionic interaction to the BE estimated for the IL 37 SO2
system (see Fig. 6). The adsorption of SO2 by IL 37 is mainly
carried out through O (anion) S(SO2) interaction (labelled as d5).
Even if, SO2 molecule also owns two intermolecular O H bonds
with alkyl H atoms located in the cation (d6 and d7). In
concordance with ion contributions to |BE 0 |, anion SO2
interaction (based on its larger electronic density value) is
greater than cation SO2 interaction.
Set V (IL 41 44) comprises those ILs whose anions have at least
one CN group, i.e., thiocianate ([SCN]) and dicyanamide ([DCA]).
ILs based on the imidazolium cation (41–43) supply KH C 4.23 105 Pascal and |BE0 | C 76.21 kJ mol1, while [BMPyr][DCA] (IL 44)
yields KH = 2.88 105 Pascal and |BE0 | = 68.20 kJ mol1. Isolated
ions provided |BE| = 78.40 kJ mol1 and 61.29 kJ mol1, while
|BE| for imidazolium and [BMPyr] + are C 36.99 kJ mol1 and
This journal is © the Owner Societies 2015
31.70 kJ mol1, respectively. Anew, the trend perceived for the
interaction between anions (cation) and SO2 in the absence of
the cation (anion) is also found for both ion SO2 contributions
to the binding energies. [EMIM][SCN] (41) brings a |BE 0 | similar
to anion SO2 contribution, while |BE 0 | for [DCA] based ILs
comes from both ion SO2 contributions. Fig. 14 and Table 6
reports optimized geometries for [EMIm][SCN] (41), [EMIm][DCA]
(42) and [BMPyr][DCA] (44). As expected (in the absence of SO2),
both ions interact with the [EMIM]+ cation through its H in
position 2. S and N terminal atoms from [SCN] anion are able
to interact with these H atoms (d1 and d2). Further, the S atom also
provides an intermolecular interaction with methyl hydrogen (d3).
Although, [DCA] owns two CN groups, only one of them interacts
with H in position 2 (d8), even though two interactions with alkyl
H atoms are also found (d9 and d10). Regarding IL 44, only one N
group interacts with the main position provided by the cation
(d14), although other intermolecular H bonds (with lower r) are
also present (d15–d18). According to electronic density values, ionic
interactions are stronger for [SCN] anion, which agrees with its
higher |BE| vales (see Fig. 6). ILs 41, 42 and 44 show similarities
regarding to the interactions with the gas molecule. Thus, the
main interaction is carried out between one terminal N (anion)
and the central S atom (d5, d11 or d19 for IL 41, 42 or 44,
respectively). Although electronic density for d5 (0.387 a.u.) is
smaller than electronic density for d11 and d19 (C0.423 a.u.),
larger charge transfer from the [SCN] anion up to the gas
(see Fig. 8) agrees with greater [SCN] SO2 contribution in
Fig. 14 Optimized geometries of [EMIM][SCN] (41), [EMIM][DCA] (42) and
[BMPyr][DCA] (44). Main intermolecular interactions are also displayed.
Atom colour code: C (gray), oxygen (red) sulphur (yellow), hydrogen
(white), nitrogen (blue) and fluorine (light blue). See Table 6 for a more
detailed description on intermolecular interactions.
Phys. Chem. Chem. Phys., 2015, 17, 13559--13574 | 13571
View Article Online
Paper
PCCP
Table 6 Intermolecular distances (d) along the electronic density values
(r) of [EMIm][SCN] (41), [EMIm][DCA] (42) and [BMPyr][DCA] (44). See
Fig. 14 for labeling
Table 7 Intermolecular distances (d) along the electronic density values (r)
of [EMIm][Cl] (45), [EMIm][Br] (48) and [EMIM][I] (51). See Fig. 15 for labeling
IL SO2
IL
IL SO2
IL
Open Access Article. Published on 20 April 2015. Downloaded on 06/02/2017 10:22:27.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
d/Å
41 – [EMIm][SCN]
2.567
d1
2.907
d2
2.841
d3
2.532
d4
d5
d6
d7
42 – [EMIm][DCA]
2.348
d8
2.180
d9
2.161
d10
d11
d12
d13
44 – [BMPyr][DCA]
d14
2.750
2.462
d15
2.689
d16
2.550
d17
2.294
d18
d19
d20
d21
d/Å
r/a.u.
d/Å
r/a.u.
0.0152
0.0142
0.0092
0.0093
2.572
0.0141
2.369
2.506
2.513
0.0387
0.0089
0.0107
2.480
2.395
2.535
2.431
2.376
2.630
0.0115
0.0051
0.0048
0.0430
0.0118
0.0074
2.745
2.406
2.534
2.585
2.463
2.464
2.617
2.448
0.0088
0.0097
0.0117
0.0103
0.0105
0.0105
0.0103
0.0088
0.0137
0.0177
0.0181
0.0084
0.0106
0.0075
0.0104
0.0143
Fig. 15 Optimized geometries of [EMIm][Cl] (45) and [EMIM][Br] (48).
Similar geometries are obtained for [EMIM][I] (51). Main intermolecular
interactions are also displayed. Atom colour code: C (gray), oxygen (red)
sulphur (yellow), hydrogen (white), nitrogen (blue), chloride (green) and
bromide (garnet). See Table 7 for a more detailed description on intermolecular interactions.
IL41–SO2 system (Fig. 7). The SO2 molecule also interacts
(through both hydrogen atoms) with the cation (d6 and d7,
d12 and d13 or d20 and d21 for IL 41, 42 or 44, respectively).
Instead the selected IL, the sum of the electronic density for
both intermolecular bonds is C0.0190 a.u. Thus, cation SO2
contribution to the BE is similar for all ILs within set V.
13572 | Phys. Chem. Chem. Phys., 2015, 17, 13559--13574
45 – [EMIm][Cl]
1.982
d1
2.750
d2
d3
d4
d5
48 – [EMIm][Br]
2.798
d6
2.865
d7
2.861
d8
d9
d10
d11
51 – [EMIm][I]
d6
2.987
3.041
d7
3.075
d8
d9
d10
d11
r/a.u.
d/Å
r/a.u.
0.0264
0.0096
2.515
2.681
2.589
2.668
2.367
0.0149
0.0111
0.0474
0.0110
0.0115
0.0242
0.0101
0.0095
2.581
0.0157
2.754
2.709
2.112
2.321
0.0117
0.0455
0.0189
0.0125
0.0218
0.0088
0.0099
2.753
0.0149
2.963
2.775
2.121
2.383
0.0095
0.0455
0.0182
0.0134
Set VI is devoted to those ILs based on imidazolium cations and
halides (IL 44–55). Keeping constant the halide, [EMIM]+ cation
always provides the lowest KH values, while for the same cation KH
increases from chloride to bromide. Similar trends are noted for
|BE0 | (see Fig. 7), i.e., high KH is related with low |BE0 |. The elected
halides in this work gave |BE| values (C52.18 kJ mol1) lower than
other anions; even though this |BE| is larger than those obtained
for imidazolium cations (C35.99 kJ mol1). For ILs 45–55,
cation SO2 and anion SO2 contributions take values of around
8.00 kJ mol1 and 88 kJ mol1. Halide effects on the SO2 adsorption mechanism have been analyzed for ILs based on the [EMIM]+
cation as a function of the anion. Optimized structure for
[EMIM][Cl]and [EMIM][Br] (optimized structures for [EMIM][[I] is
not displayed since similar results to [EMIM][Br] are obtained) are
shown in Fig. 15, while the main structural parameter of intermolecular interactions along their electronic density values are
collected in Table 7. As expected, the main interaction between
both ions is a hydrogen bond between the halide and the H atom
located in position 2 (d1 or d6 for IL 45 or 48/51, respectively),
which is weakened in the presence of SO2 molecule. For IL SO2
systems, S X (X = Cl, Br or I) is the main interaction (labelled as
d3 or d9 for IL 45 or 48/51, respectively), while two O H
intermolecular bonds are also found between SO2 and the cation.
As seen in Table 6, electronic density for d6 is much greater than
those of cation SO2 interactions in concordance with its larger
contribution from [Cl] SO2 interaction. The same behaviour is
also noted for ILs 48 and 51.
4. Conclusions
This contribution reports a density functional theory (DFT) on
several ILs, for which high SO2 solubility is expected. This work
This journal is © the Owner Societies 2015
View Article Online
Open Access Article. Published on 20 April 2015. Downloaded on 06/02/2017 10:22:27.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
PCCP
is divided into three parts: (i) we selected a set of ILs which
should provide high efficiency for SO2 capture. For this, a
screening of a large number of ILs via the COSMO-RS method
was done; (ii) binding energies between SO2 and ILs were
analyzed intensely through DFT simulations for a set of 55 ILs,
which provided high efficiency in SO2 capture according to the
COSMO-RS method; (iii) intermolecular interaction for some
representative ILs were deeply studied through the AIM theory
aimed at obtaining some information on the SO2 adsorption
mechanism at the molecular level. The results evidenced the
ability of the selected cations and anions to interact with the
SO2 molecule, which is stronger for anion SO2 interactions.
Thus, anion SO2 interactions are ruled by a strong charge
transfer from the anion to SO2 molecule. For the ILs SO2
system, the total binding energy (BE) has been decomposed in
the contributions from the interactions between ions,
anion SO2 and cation SO2. The interaction between both
ions always provided the largest contribution to the total
binding energy. Then, the binding energy related with SO2
capture by ILs was also calculated considering the ILs as a
whole (BE 0 ). A value of around 36.58 kJ mol1 (for CO2 capture
by [EMIM][NTf2] IL, which was taken as a pivotal reference
for comparison purposes) as a low limit; all ILs yield
larger binding energies. Most of them provide values of
around 45.0 kJ mol1. Therefore, all of them would provide
high SO2 capture efficiency. Through the comparison between
ion SO2 contributions and BE 0 , we could obtain some
information on what ions mainly govern the SO2 capture
within the ILs. In most cases, SO2 capture would be mainly
ruled out by the anion or by both ions. Even if the SO2 capture
mechanism at the molecule level depends on each ILs,
some common features as found for related ions. Even
though, a statistical analysis of binding energies of IL–SO2
systems as a function of ion–SO2 and cation–anion ones
brings to light that SO2 adsorption by ILs at the molecular
level is mainly ruled by anion–SO2 interaction and cation–
anion as well. Thus, qualitative trends on SO2 capture by ILs
can be obtained only based on the study of anion–SO2 and
isolated ILs systems. Systematic research on ILs for SO2
capture allow increase of our knowledge about those factors
which could be controlled at the molecular level, allowing an
approach up to the rational design of task-specific ILs for
future applied studies.
Acknowledgements
Gregorio Garcı́a acknowledges the funding by Junta de
Castilla y León (Spain), cofunded by European Social Fund,
for a postdoctoral contract. Santiago Aparicio acknowledges
the funding by Ministerio de Economı́a y Competitividad
(Spain, project CTQ2013-40476-R) and Junta de Castilla y León
(Spain, project BU324U14). Mert Atilhan acknowledges support
of an NPRP grant (No: 6-330-2-140) from the Qatar National
Research Fund. The statements made herein are solely the
responsibility of the authors.
This journal is © the Owner Societies 2015
Paper
References
1 S. J. Smith, J. van Aardenne, Z. Klimont, R. J. Andres,
A. Volke and S. Delgado Arias, Atmos. Chem. Phys., 2011,
11, 1101–1116.
2 G. Cui, C. Wang, J. Zheng, Y. Guo, X. Luo and H. Li, Chem.
Commun., 2012, 48, 2633–2635.
3 J. Huang, A. Riisager, P. Wasserscheid and R. Fehrmann,
Chem. Commun., 2006, 4027–4029.
4 Z. Z. Yang, L. N. He, Y. N. Zhao and B. Yu, Environ. Sci.
Technol., 2013, 47, 1598–1605.
5 D. Yang, M. Hou, H. Ning, J. Ma, X. Kang, J. Zhang and
B. Han, Reversible Capture of SO2 through Functionalized
Ionic Liquids, ChemSusChem, 2013, 6(7), 1191–1195.
6 S. Tian, Y. Hou, W. Wu, S. Ren and C. Zhang, RSC Adv.,
2013, 3, 3572–3577; X. L. Yuan, S. J. Zhang and X. M. Lu,
J. Chem. Eng. Data, 2007, 52, 596–599; S. Ren, Y. Hou,
W. Wu, Q. Liu, Y. Xiao and X. Chen, J. Phys. Chem. B,
2010, 114, 2175–2179; G. Cui, J. Zheng, X. Luo, W. Lin,
F. Ding, H. Li and C. Wang, Angew. Chem., Int. Ed., 2013, 52,
10620–10624.
7 G. Yu and X. Chen, J. Phys. Chem. B, 2011, 115, 3466–3477.
8 S. Ren, Y. Hou, S. Tian, X. Chen and W. Wu, J. Phys. Chem. B,
2013, 117, 482–2486.
9 Z. Lei, C. Dai and B. Chen, Chem. Rev., 2013, 114, 1289–1326.
10 S. Aparicio and M. Atilhan, Energy Fuels, 2010, 24,
4989–5001; S. Aparicio and M. Atilhan, J. Phys. Chem. B,
2012, 116, 9171–9185; F. Karadas, M. Atilhan and S. Aparicio,
Energy Fuels, 2010, 24, 5817–5828; D. H. Zaitsau, A. V.
Yermalayeu, S. P. Verevkin, J. E. Bara and A. D. Stanton,
Ind. Eng. Chem. Res., 2013, 52, 16615–16621; C. Wang, X. Luo,
H. Luo, D. E. Jiang, H. Li and S. Dai, Angew. Chem., Int. Ed.,
2011, 50, 4918–4922; X. Zhang, H. Dong, Z. Zhao, S. Zhang
and Y. Huang, Carbon capture with ionic liquids: overview
and progress, Energy Environ. Sci., 2012, 5, 6668–6681.
11 F. Karadas, B. Köz, J. Jacquemin, E. Deniz, D. Rooney,
J. Thompson, C. T. Yavuz, M. Khraisheh, S. Aparicio and
M. Atilhan, Fluid Phase Equilib., 2013, 351, 74–86.
12 V. Sanz, R. Alcalde, M. Atilhan and S. Aparicio, J. Mol.
Model., 2014, 20, 1–14.
13 S. Aparicio, M. Atilhan, M. Khraisheh, R. Alcalde and
J. Fernández, J. Phys. Chem. B, 2011, 115, 12487–12498.
14 J. Palomar, M. González-Miquel, A. Polo and F. Rodrı́guez,
Ind. Eng. Chem. Res., 2011, 50, 3452–3463.
15 J. Earle-Martyn and R. Seddon-Kenneth, Green Solvents for
the Future, in Clean Solvents, American Chemical Society,
2002; J. S. Wilkes, Green Chem., 2002, 4, 73–80; R. D. Rogers
and K. R. Seddon, Science, 2003, 792–793.
16 O. Hollóczki, Z. Kelemen, L. Könczöl, D. Szieberth,
L. Nyulászi, A. Stark and B. Kirchner, ChemPhysChem,
2013, 14, 315–320; K. M. Gupta and J. Jiang, J. Phys. Chem.
C, 2014, 118, 3110–3118; P. Gu, R. Lü, S. Wang, Y. Lu and
D. Liu, Comput. Theor. Chem., 2013, 1020, 22–31; F. Yan,
M. Lartey, K. Damodaran, E. Albenze, R. L. Thompson,
J. Kim, M. Haranczyk, H. B. Nulwala, D. R. Luebke and
B. Smit, Phys. Chem. Chem. Phys., 2013, 15, 3264–3272;
Phys. Chem. Chem. Phys., 2015, 17, 13559--13574 | 13573
View Article Online
Paper
17
Open Access Article. Published on 20 April 2015. Downloaded on 06/02/2017 10:22:27.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
18
19
20
21
22
23
C. Wu, T. P. Senftle and W. F. Schneider, Phys. Chem. Chem.
Phys., 2012, 14, 13163–13170; B. Gurkan, B. F. Goodrich,
E. M. Mindrup, L. E. Ficke, M. Massel, S. Seo, T. P. Senftle,
H. Wu, M. F. Glaser, J. K. Shah, E. J. Maginn, J. F. Brennecke
and W. F. Schneider, J. Phys. Chem. Lett., 2010, 1, 3494–3499.
G. B. Damas, A. B. A. Dias and L. T. Costa, J. Phys. Chem. B,
2014, 118, 9046–9064.
A. Klamt, COSMO-RS: From Quantum Chemistry to Fluid
Phase Thermodynamcis and Drug Design, Elsevier, Amsterdam,
2005; F. Eckert and A. Klamt, AIChE J., 2002, 48, 369–385.
M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato,
X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng,
J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda,
J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,
H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta,
F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin,
V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari,
A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi,
N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross,
V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E.
Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli,
J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski,
G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich,
A. D. Daniels, ö. Farkas, J. B. Foresman, J. V. Ortiz,
J. Cioslowski and D. J. Fox, Gaussian 09, Revision D.01,
Gaussian, Inc., Wallingford, CT, USA, 2009.
C. Lee, W. Yang and R. G. Parr, Phys. Rev. B: Condens. Matter
Mater. Phys., 1988, 37, 785–789; A. D. Becke, J. Chem. Phys.,
1993, 98, 5648; A. D. Becke, Phys. Rev. A: At., Mol., Opt. Phys.,
1988, 38, 3098–3100.
A. J. Cohen, P. Mori-Sánchez and W. Yang, Chem. Rev., 2012,
112, 289–320.
S. Grimme, J. Comput. Chem., 2006, 27, 1787–1799.
T. Schwabe and S. Grimme, Phys. Chem. Chem. Phys., 2007, 9,
3397–3406; S. Zahn and B. Kirchner, J. Phys. Chem. A, 2008,
112, 8430–8435; S. Grimme, W. Hujo and B. Kirchner, Phys.
Chem. Chem. Phys., 2012, 14, 4875–4883; E. I. Izgorodina,
U. L. Bernard and D. R. MacFarlane, J. Phys. Chem. A, 2009,
113, 7064–7072; S. Zahn, F. Uhlig, J. Thar, C. Spickermann
and B. Kirchner, Angew. Chem., Int. Ed., 2008, 47, 3639–3641.
13574 | Phys. Chem. Chem. Phys., 2015, 17, 13559--13574
PCCP
24 J. Marten, W. Seichter, E. Weber and U. Bohme, CrystEngComm, 2008, 10, 541–547.
25 S. Simon, M. Duran and J. J. Dannenberg, J. Chem. Phys.,
1996, 105, 11024; S. F. Boys and F. Bernardi, Mol. Phys.,
1970, 19, 553–566.
26 R. A. Ando, L. J. A. Siqueira, F. C. Bazito, R. M. Torresi and
P. S. Santos, J. Phys. Chem. B, 2007, 111, 8717–8719;
L. J. A. Siqueira, R. A. Ando, F. C. C. Bazito, R. M. Torresi,
P. S. Santos and M. C. C. Ribeiro, J. Phys. Chem. B, 2008, 112,
6430–6435.
27 R. S. Mulliken, J. Chem. Phys., 1955, 23, 1833.
28 J. J. Philips, M. A. Hudspeth, Jr, P. M. Browne and
J. E. Peralta, Chem. Phys. Lett., 2010, 495, 146–150.
29 C. M. Breneman and K. B. Wiberg, J. Comput. Chem., 1990,
11, 361–373.
30 I. Bandrés, R. Alcalde, C. Lafuente, M. Atilhan and
S. Aparicio, J. Phys. Chem. B, 2011, 115, 12499–12513.
31 R. F. W. Bader, Atoms in Molecules: a Quantum Theory,
Oxford, 1990.
32 T. Lu and F. Chen, J. Comput. Chem., 2012, 33, 580–592.
33 J. Palomar, V. R. Ferro, J. S. Torrecilla and F. Rodrı́guez,
Ind. Eng. Chem. Res., 2007, 46, 6041–6048; J. Palomar,
J. S. Torrecilla, V. R. Ferro and F. Rodrı́guez, Ind. Eng. Chem.
Res., 2009, 48, 2257–2265; A. Klamt, Wiley Interdiscip. Rev.:
Comput. Mol. Sci., 2011, 1, 699–709; M. A. Diedenhofen and
A. Klamt, Fluid Phase Equilib., 2010, 294, 31–38;
X. Zhang, Z. Liu and W. Wang, AIChE J., 2008, 54,
2717–2728; M. B. Miller, D. L. Chen, H. B. Xie,
D. R. Luebke, J. Karl Johnson and R. M. Enick, Fluid Phase
Equilib., 2009, 287, 26–32; R. Franke, B. Hannebauer and
S. A. Jung, Chem. Eng. Technol., 2010, 33, 251–257;
N. A. Manan, C. Hardacre, J. Jacquemin, D. W.
Rooney and T. G. Youngs, J. Chem. Eng. Data, 2009, 54,
2005–2022.
34 M. González-Miquel, J. Palomar and S. Omar, Ind. Eng.
Chem. Res., 2011, 50, 5739–5748.
35 M. González-Miquel, J. Bedia, J. Palomar and F. Rodrı́guez,
J. Chem. Eng. Data, 2014, 59, 212–217.
36 M. Atilhan, J. Jacquemin, D. Rooney, M. Khraisheh and
S. Aparicio, Ind. Eng. Chem. Res., 2013, 52, 16774–16785.
37 G. Garcı́a, M. Atilhan and S. Aparicio, Chem. Phys. Lett.,
2014, 610, 267–272.
This journal is © the Owner Societies 2015